Stents are tubular molds which are made of biocompatible materials and are contracted upon their introduction and then they dilate for their insertion into the desired duct or lumen. Some types of stents dilate mechanically, and they require for their insertion the use of an expandable element arranged along the introducing instrument and dilatable inside the stent.
As the instrument for the introduction of mechanically dilating stents, a catheter having an inflatable balloon at its distal end of the type already used may be used, e.g., for the dilation of arterial ducts or other lumina in a live body. Therefore, for the introduction operation, the balloon, empty, is wrapped tightly around a corresponding zone of the catheter, and the stent, in its turn, is arranged tightly around the balloon in order to remain fixed there during the introduction into a lumen.
However the requirements of a catheter for the simple dilation of ducts or lumina most often are incompatible with those of a catheter for the positioning of a stent, which is why specific catheters would be needed for one or the other operation.
A dilating catheter, for example, for angioplasty procedures, or the like, must have a small diameter, a highly resistant balloon, and a low coefficient of surface friction. On the other hand a catheter for positioning stents must have a remarkable friction at least at the balloon, must be made of a material with a moldable surface that is adapted to the interior of the closed, i.e., contracted stent, and must have a diameter of the balloon, when closed around the catheter, at least slightly greater than the internal diameter of the closed stent. All this is to interfere with the stent and to hold it, preventing its loss during the insertion in a duct or lumen.
In other words, the surface slipperiness of the balloon may be a cause of a loss of the stent.
In addition, a diameter in the zone of the catheter, including the closed balloon wrapped there, that is smaller than the internal diameter of the closed stent that is applied there may cause:
a so-called overlapping resulting from a corrugation and an overlapping of some parts of the stent with possible deformation of its structure, if the stent is contracted too tightly in order to make it adhere to such a zone; or
an improper fixation of the stent with the possibility still of losing it during the introduction if the stent, although suitably and correctly closed, does not adhere to the outer surface of the catheter plus closed balloon.